This invention relates to printed matters being printed by the use of such a kind of ink as containing pH indicator for its ingredient, hereinafter referred to as "coloring ink" which ink is able to develop color under the action of a coloring assistant and to lose color in a short time with the passage of time, and, if necessary, it can be made respectedly, to develop color and lose color any number of times.
As printed matters where their printed parts being printed invisibly ahead of time are made to become visible by applying some liquid agent thereto with painting means such as, for example, a pen or a brush, there have been heretofore proposed such kinds of things as disclosed in Japanese Utility Model Publication No. 52063 of 1981, and further various contrivances regarding painting means or printing inks are described, for example, in Japanese Patent Application Disclosure No. 135720 of 1976, and others.
However, conventional ones as, for example, in the above Utility Model Publication No. 52063 of 1981 contain zinc chloride for preventing the coloring from growing weak with the passage of time and in the end losing color, and thereby are so contemplated only that after the invisible printed part once develops color, the coloring remains as permanently as possible, but there is no thought at all that the coloring is permitted to lose of its own accord, in a short time after developing color. Therefore, in such printed matters as study books or question books having spaces for notes serving as spaces for answers printed abreast of spaces for questions, the coloring of the spaces for notes lasts for long hours after once having developed color visibly, even when the above spaces for notes have been printed as color-developable but invisible parts. As the results, it becomes impossible to procecute one's learning repeatedly solving the questions by returning the spaces for notes to a state of blank any number of times.
Consequently, such ones as above, in order to solve problems in a plural number of times without seeing the space for answers, is required like printed matters printed using only conventional visible inks to have the space for answers and others printed in the end of a book or in its marginal spaces or in a separate volume. Accordingly, it takes time and labor to compare questions and answers or explanations from one to another, thereby leading to the lowering of the efficiency of learning.
Of course among conventional ones, not that there is not such a one where the efficacy of ink dies away under the influence of ultraviolet rays, oxidation, etc., in company with which the colored printed part loses color of itself when being left alone for long hours, for example, in about several days, say, in about 3 days, but it is substantially impossible for such a conventional one as above to prosecute the repetition of learning able to enhance the efficiency of study by losing the color of the space for answers in a short time, that is, in a minute to 12 hours, preferably in about 2 to 10 minutes, in as much as it is required long time more than several days for its fading.
Further, conventional ones have no intention from the first to develop color and to lose color over and over again, and consequently they are hard to be used repeatedly.